1. Field of the Invention
The present invention relates to systems and methods for calibrating the pointing of antennas used in satellite-based navigation, and in particular, to a method and apparatus for calibrating such antennas by use of scanned spot beam signal characteristics measured by a plurality of navigation receivers.
2. Description of the Related Art
The Global Positioning System (GPS) is a satellite system that transmits navigation signals that are received by navigation receivers and used to determine the position of the receiver to a high degree of accuracy. Currently, GPS provides standard service to commercial receivers, and a higher accuracy signal to military receivers designed to receive them.
Since such signals have military applications, countermeasures can be expected to be applied to reduce their effectiveness. One such countermeasure is jamming. To increase the effectiveness of the GPS signals in a jamming environment, a high gain spot beam antenna may be used to beam high intensity spot beams of GPS signals to areas where such jamming is expected to occur. However, the high gain of such antennas is obtained at the cost of reduced beamwidth.
Improperly pointing a spot beam antenna can result in many significant and very costly problems. The pointing problem means that GPS users (e.g., GPS-based weapon systems) in the targeted area of operation may not receive the spot beam GPS signal (rendering the weapon useless), and other equipment in the neighboring region may be unintentionally disrupted by this high-gain signal. This problem will also result in greater collateral damage and an increased number of sorties and weapons. Accurate antenna pointing needs to be maintained within a tight bound to support the Area of Operations and to minimize radio frequency interference in the neighboring region.
Hence, for such a system to be effective, the spot beams must be accurately directed at the area of operation (AOO) of interest. To achieve this, the space vehicle attitude sensors and antenna signal boresight must be accurately calibrated.
Techniques for calibrating the antenna signal boresight can be categorized as either ground calibration or on-orbit calibration. In ground calibration, the antenna signal boresight and other satellite systems ate physically calibrated on the ground during pre-flight tests. While effective, the ground calibration approach renders inaccurate solutions due to launch vehicle vibration and other effects in between the calibration on the ground and the final activation of the satellite in space.
The second category of calibration is on-orbit calibration. This technique has been applied to geosynchronous and other types of satellites, and uses ground stations and fixed-location ground beacons which are dedicated for that purpose. Such a system for interplanetary spacecraft is disclosed in “In-flight Calibration Technique for Onboard High-Gain Antenna Pointing”, Hiroshi Ohtakay and Jerome Hardman, AIAA Journal of Spacecraft, Vol. 12 No. 12, page 754-759, which is hereby incorporated by reference herein. Unfortunately, this technique requires large fixed ground antennas which have limited availability due to resource sharing among different satellite constellations. Such resource sharing isn't a difficult problem for geosynchronously based constellations, but typical navigation satellites such as GPS satellites are not geosynchronous, and do not maintain a fixed position over a point on the Earth, but are constantly moving relative to the surface of the Earth. They thus may require additional ground stations, more frequent calibration, further burdening the ground resources.
The ground antennas are also dedicated to the purpose of calibration, and are costly. In addition, due to a limited number of fixed ground antenna locations, the geometry of the satellite and ground system often limits the effectiveness of such on-orbit systems. Such problems are worse for non-geosynchronous satellites, as they are moving with respect to the Earth's surface.
What is needed is a method for inexpensively and quickly calibrating the antenna boresights without the need for expensive ground stations dedicated to that purpose. The present invention satisfies this need.